Thevenin Equivalent Resistance for a Black Box

1. The problem statement, all variables and given/known data
I am trying to find the theoretical Thevenin equivalent resistance for a black box that I used in an experiment. The measured Thevenin equivalent resistance was roughly 4786[tex]\Omega[/tex]. However, I have tried several times to solve the problem, but keep arriving at a theoretical value of around 3511[tex]\Omega[/tex]. That just seems to be a little too low...

I have attached the circuit schematic, with resistor values included.
Could someone please look at the problem and see if I am on the right track. If so, any pointers on how I should finish it?

By the way, notice that R2, R3 and R4 are all three in parallel. If you replace then by their equivalent, and replace R5 and R6 by an equivalent, you will have a simple ladder network. You can then work that out very easily, without any need for delta-y or similar transformations.

I assume you got your measured value in the lab with a simple ohmmeter measurement. Did you remember to replace the DC voltage with a short circuit before you made your measurement?

Thanks for all of your advice.

I came to my measured value of R_th by measuring the open circuit DC voltage between the terminals of the black box (V_th) and then measuring short circuit DC current between the terminals of the black box (I_sc)
then calculating R_th = (V_th)/(I_sc)...